Sex that is a direct result of a man's sexual partner/ex-partner bragging to friends, relatives, co-workers, and various other heifers and hos, about the sexual skills or size of her hook-up/boyfriend/husband.
The corollary for women is 'referral cock'.
John: Man, I got some sweet referral pussy last night.
Bob: Whaaa? No shit?
John: No shit, I went to the bar and yo mama asked me if it was true that I could eat a pussy like a fat kid at a pie eatin' contest. One of your sisters musta told her.
Bob: Shut up!